An in vitro assay for protein synthesis and breakdown has been developed by incubating weanling rats' extremity muscles in a physiological buffer system. Test sera are added from normal, septic and cachectic animals to determine if there is a factor in the sera which decreases protein synthesis and increases protein catabolism. Such a circulating factor has been reported in the septic state. An in vivo assay has been developed for a cachectic factor by administering sera from cancer bearing animals who are not eating to normal animals, and measuring food intake. If these studies demonstrate a factor, additional studies will be done to determine if it is a tumor derived factor or a host derived factor in response to the tumor, and to biochemically characterize the factor. Exogenous insulin has been administered to rats bearing a sarcoma. Insulin given to tumor-bearing animals increases spontaneous food intake, nitrogen balance, body weight gain, decreases muscle catabolism (3 methyl histidine) without stimulating tumor growth. Survival studies show no advantage to the insulin treated animals in time of survival but the animals have greater carcass mass. Compositional analysis indicates that the accrual is both protein and fat similar to normal body composition. Thus, exogenous insulin appears to preserve the host without feeding the tumor.